As pointed out in an unpublished paper of Chern, Djordjevic and Barnett, entitled Ultrasonic Method for Non-intrusive low liquid level Sensing (incorporated herein by reference and attached to the invention disclosure statement submitted with this application), the Physics and fundamental principles of ultrasonic pulse echo technology are well understood, and pulse echo techniques have been used widely for measurement of material properties such as modulus, stress and thickness, and for defect detection and flaw characterization. The Chern et al paper describes the application of the pulse echo principle to a non-intrusive method of low-liquid-level sensing for potential application in space systems. Chern et al point out that the ultrasonic approach to low-liquid sensing provides a viable alternative to conventional approaches which require penetration of the container wall and special designs to accommodate the sensor and to access the liquid.
In a second unpublished paper of Eric Johnson of Martin Marietta (incorporated herein by reference, a copy of which is attached to the information disclosure statement submitted with this application), the author refers to prior use of a method for determining when a launch vehicle liquid fuel tank is empty. As pointed out by the author, an Ultrasonic transducer (sender/receiver) is mounted on the outside wall of a fuel tank. If the tank is empty, a pulse train stimulated by the transducer is reflected back and forth between the inner and outer surfaces of the tank walls. This pulse train is attenuated each time it is reflected from the inner or outer surface of the material comprising the walls. The signal detected by the transducer is, therefore a decaying echo train. If, however the tank contains enough liquid to cover the section of the inner wall opposite the transducer, there is a greater attenuation of the pulse train. By comparing the attenuations of the pulse trains, a determination can be made as to when the tank is empty. However, the author points out, that if the fuel is dispensed very rapidly, as in liquid fuel rocket applications, a residual film may adhere to the tank wall and this film effect the determination of when the tank is empty.
It is noted that the Chern et al method requires the use of a peak detector which makes the detector fairly sensitive to noise in the frequency band being used, and second, if a time gate is used for the peak detector, the number of detected peaks depends on the thickness of the tank walls. It is also noted from the Johnson paper, that peak detection is enormously sensitive to wave interference in the presence of thin liquid layer, and, by inference, to other uncontrolled reflections. It is noted that the Johnson implementation of the sensor uses a logic circuit which requires four successive state determinations to be the same before the state of the liquid in the tank is determined to be correct.
In accordance with the invention described and claimed herein, I use a method which avoids the difficulties of the prior art by integrating the whole pulse train. As will be shown, the integral depends only on the transmitted pulse magnitude and duration, and on the loss experienced on each round trip. This integral method will detect the return signal and integrate for a fixed time sufficient to get all the return pulses of appreciable magnitude. The integral will be sensitive only to the amount of energy returning from the liquid, and independent of the wave interference is so devastating to a peak detector.